The present invention relates to the field of data and voice frequency communications. By way of example, the present invention is illustrated as a method and apparatus for transmitting and receiving an integrated services digital network (ISDN) 2B+D signal and voice channel over a single twisted pair telephone line.
Techniques for the transmission of multiple voice or data signals via ISDN are well known in the telecommunications industry. An increasing amount of subscribers often desire the use of such conventional ISDN for electronic data processing equipment such as telephones, computers, facsimile machines, modems, and the like in order to take advantage of video conferencing, Internet browsing and access as well as telephone service. However, telephone companies have been reluctant to offer ISDN services for a number of reasons.
One limitation with the provision of conventional ISDN service is that standard ISDN equipment (NT1 device) may experience failure and then leave subscribers without plain old telephone services (POTS). Standard ISDN equipment provides 2B+D (Basic Rate ISDN framing format with 2 basic or bearer (B) channels and 1 data (D) channel) digital services, with both B channels being used to provide data, both B channels being used to provide voice channels, or one B channel being used to provide data and another B channel being used to provide a voice channel. Standard ISDN equipment may thus be used to provide voice telephone service in addition to other digital services, but failure of the ISDN equipment results in failure of all services including the voice telephone service. Therefore, when ISDN service fails, digital telephone service fails with it. It is often desired to have POTS at a customer/subscriber premises as a "back-up " or "lifeline" service when the conventional ISDN service fails due to ISDN equipment failure or a power outage at the subscriber premises. However, providing this analog phone service conventionally requires adding an additional copper twisted pair line onto a subscriber premises, which may be difficult, costly, and time consuming. That is, because copper is a limited and expensive telephone company resource that may not be available at the desired customer premises, adding an analog telephone line may require breaking concrete and laying down additional copper cable which requires great time and expense.
From the above, it is seen that a method and apparatus for providing ISDN service and voice "lifeline" telephone service from a telephone company central office to a subscriber premises over pre-existing copper wires is desirable.